High speed power semiconductor switching in variable frequency motor drive (VFD), soft motor starting systems, and power electronic systems do not generate sinusoidal output voltage waveforms, but instead generate a continuous train of high frequency pulses. These voltage pulses are transmitted to the motor terminals through the motor cables connected to the variable frequency motor drive. Peak pulse voltage at the converter terminal is equal to the drive DC bus magnitude (VDCbus) and contains a steep fronted rise and fall times [PWM] controlled by a semi-conductor switch device such as an Insulated Gate Bipolar Transistor (IGBT) or the like. The peak pulse voltage at the motor terminals is typically not VDCbus, but is dependent on the transfer characteristics of the entire circuit including the converter, conductor, and the load. The load terminal voltage phenomenon is sometimes called “transmission line effect”, “reflected electromagnetic waves”, or “standing electromagnetic waves.”
The peak voltage pulse can be defined in terms of a ratio of the peak line motor voltage to peak line bus voltage. In some systems, per unit (pu) voltage can be up to 2 or greater at certain lengths of cable regardless of the type of output switching device. High frequency reflected transients are generated when the surge impedance of the converter(s), cable(s) and motor(s) are not matched. If not attenuated, the generated voltage transients can damage the insulation in the electrical system.
One cause of over-voltages at drive and motor terminals is voltage reflections. Voltage reflections occur when characteristic impedances are mismatched and the traveling time of a transient along a transmission line is greater than one tenth of the transient's rise time, i.e., “electrically long.” If this transient is not damped or otherwise attenuated, it will produce an over-voltage at the mismatch. In addition, the mismatch can produce standing waves in the transmission line.
Most power electronic inverters transmit power as differential mode content. This means that the sum of the power frequency currents is essentially zero. However, switching typically appears as common mode content. Since the sum of the three phase voltages or currents are not equal to zero, the return path is through the common mode path which is the cable shield, cable drain wire, or the earth.
Present solutions for attenuating high frequency transients include large, heavy and costly low pass filters such as sine wave filters to prevent reflected waves. Accordingly, there remains a need for further contributions in this area of technology.